Saturated polyester, such as polyethylene terephthalate (PET) or polybutylene terephthalate (PBT), is a linear thermoplastic polymer comprising ester bonds in a main chain. Since the saturated polyester is advantageous in dimension stability, weather resistance and surface smoothness and has high transparency and glossy appearance, it has been widely used as a material for various molded articles such as synthetic fibers, films, containers, housings, etc.
However, the saturated polyester has disadvantages that since it exhibits low glass transition temperature (Tg), poor heat resistance, and gas permeability to some gases, it cannot be used as a packaging material for fruit beverages, beers, green tea products, rice beverages, etc.
In order to solve these disadvantages, polyethylenenaphthalate (PEN) resin and a mixed polymer of polyethyleneterephthalate and polyethylenenaphthalate have been proposed. These products are currently in use. However, the price of polyethylenenaphthalate (PEN) resin is high relative to polyethyleneterephthalate and thus is economically disadvantageous. Moreover, it is difficult to recycle the polyethylenenaphthalate resin. Alternatively, a method for improving heat resistance, transparency and gas impermeability is disclosed in Japan Patent Laid-open No. 1997-290457. In accordance with this method, the crystal orientation of polyester is enhanced by performing biaxial stretching during molding of PET bottle. However, some problems of this method are that the degree of crystal orientation cannot be raised to above 40%, and it is not usable in the case of charging beverage into PET bottle at high temperature above 92° C. Furthermore, in the case of charging at low temperature, the productivity drops.